It is realized that there are many types of gas-liquid contacting operations presently employed in connection with various petroleum and chemical processing systems, as well as various types being provided for the scrubbing of power plant stack gases and other contaminated gaseous streams to prevent air pollution. Also, in connection with certain types of gas scrubbing operations it has been found advantageous to make use of one or more stages of lightweight mobile elements to provide contacting surfaces and effect enhanced mixing for the countercurrently flowing streams. For example, two different, but related, types of "floating bed" scrubbers make use of lightweight mobile elements in one or more spaced beds to provide improved particle removal and/or gas scrubbing operations. A U.S. Pat. No. 3,122,594, issued to A. W. Kielback, teaches the utilization of the lightweight elements in one or more vertically spaced zones of a contact chamber where the upward gas flow causes the bed of elements to float and be lifted against an upper perforate barrier where they will move and rotate from the stream flows. In U.S. Pat. No. 3,350,075, issued to H. R. Douglas, there is disclosed a gas-liquid contacting operation with less resistance to gas flow where the lightweight floating elements will provide a more turbulent, random movement between more widely spaced perforate barrier means, as compared to the more compact bed of Kielback.
Commercial installations for both of the aforedescribed contacting systems have customarily utilized lightweight hollow spheres of polyethylene and polypropylene that are about 11/2 inches in diameter and thus like the well-known "ping-pong" ball. Unfortunately, these balls are rather brittle and non-deformable so as to be subject to rapid loss of usefulness by reason of abrasion wear and breakage. The wear is particularly fast in scrubbers for flue gas streams containing fly ash and/or where limestone has been added to the liquid stream to serve as an adsorbent for SO.sub.2 removal from the gaseous stream. The cracking of contact elements results from embrittlement and from their impact against walls and grids of the scrubber, as well as against each other, and it frequently occurs at the seams which are formed during their blow-molding manufacturing procedures. Over a period of time there can also be sufficient abrasion to the thin wall of a sphere so as to cause the development of a small hole which can lead to the hollow element being filled with liquid such that it can no longer float in a desired aerated or random type of movement. Inasmuch as the shutting down of a large flue gas scrubber in a clean-up system or a contact tower in a processing plant can be highly uneconomical and/or lead to air pollution problems, it has become necessary to find better and longer life contact elements for these various scrubbing, adsorbing, or processing systems. The aforementioned patents suggest other materials for the contact elements, e.g. foamed plastics or other porous materials; however, there can be problems with most of these types of materials and it has been expedient up to the present time to primarily use the hollow polypropylene or polyethylene spheres.
In connection with research work carried out to obtain better elements, the recent test work has shown that the relatively hard, non-deformable and non-flexible materials tend to wear and break faster by shattering or by mold seam cracking than the more elastic or rubber-like materials. Thus, it may be considered a principal object of the present invention to utilize lightweight, floatable elements of a rubber-like elastomeric material in order to obtain greater wear resistance and longer life to such elements in countercurrent gas-liquid operations.
A further object of the invention is to provide lightweight, aeratable contact elements which comprise a soft, spongy and closed-cell type of elastomeric material. The spongy and elastomer properties for the resulting post cured foam elements of the present invention will have deformability and flexibility which will permit such elements to, in turn, flex and result in a continuous or periodic removal of solids materials which may tend to adhere to the surfaces of the elements. As an additional aspect, it is an object of the invention to provide lightweight elements with small closed-cells so as to substantially preclude the absorption of water or other scrubbing liquor and any loss in mobility for the elements, as well as provide the advantage of continuous wearability, as will be more fully set forth hereinafter.
In still another aspect, it may be considered a further object of the invention to provide for the use of a lightweight "solid" sphere of a closed-cell foam such that the need of a blow-molding procedure is obviated. Also, preferably, the foam spheroid will be formed from a blend which will provide "self-skinning", with a resulting relatively tough, smooth, surface that will enhance the wearability of the element in operating service. However, in the event that the tough "self-skinned" surface does wear away, by providing a lightweight closed-cell foam element, where there are quite small closed cells and a general uniformity throughout the full cross-section of a sphere, there can be the uniform wearing away of the entire surface of an element without the problem of breakage and/or any harmful liquid absorption. Actually, in a final stage of useful life there can be merely a gradual loss in weight for an element and no harm to the overall "floating bed" scrubbing operation with the multiplicity of mobile contact surfaces.
Inasmuch as contact towers for processing operations and/or for flue gas scrubbing may well contain chemically aggressive components, as well as the abrasive materials, it is generally necessary to have good chemical resistance as a desired property for a contact element. Chemical resistance should be for acids as well as bases. For example, the reactants in a gaseous stream to be scrubbed may comprise sulfur oxides, carbon dioxides, nitrogen oxides, etc., such that there may be sulfuric acid, nitric acid, hydrochloric acid, and the like, along with additive components such as lime, limestone, Dolomite, zinc oxide and gypsum. Entrained particulates and additive compounds can also result in the forming of calcium carbonate, calcium sulfate, calcium sulfite, and the like. In any event, it is to be emphasized that in certain processing and scrubbing operations, the chemical attack on the contact elements can be quite severe as well as abrasive. In still another aspect, it is, of course, desirable to have oxidation resistance for the contact elements to overcome the continuous presence of gaseous oxygen and/or ozone.